Process for the preparation of photographic emulsion

ABSTRACT

More sensitive emulsions, improved fineness of grain and improved utilization of silver are obtained in the preparation of photographic emulsions by a procedure which comprises adding a silver nitrate solution to a relatively small portion of a halogen salt solution, to which gelatin has been added and which has been heated to an appropriate temperature for the desired emulsion sensitivity, that is moved or agitated at a relatively high speed with respect to the rest of the halogen salt solution while the solution is in motion.

United States Patent inventor Horst Theilemann Rudcsheimcrstrasse 1, 8 Munich 12, Germany Appl. No. 768,021

Filed Oct. 16, 1968 Patented Dec. 21, 1971 Priority Oct. 23, 1967 Germany P 15 97 643.9

PROCESS FOR THE PREPARATION OF PHOTOGRAPHIC EMULSION 9 Claims, 6 Drawing Figs.

U.S. Cl 96/94, 96/1 14.1, 96/1 14.7, 95/89, 95/96 int. Cl G03c 1/02 Field 01 Search 96/94,

[ 56] RcierencesCited UNITED STATES PATENTS 2,618,556 11/1952 Hewitsonetal. 96/1i4.7 3,425,835 2/1969 .lohnsonetal. 96/94 OTHER REFERENCES Making and Coating Photographic Emulsions by Relikman Iet al. Focal Press Limited 1964 pp. 17- 22 and 228- 236.

Primary ExaminerWilliam D. Martin Assistant ExdminerWilliam R. Trenor Attorney-Craig, Antonelli & Hill ABSTRACT: More sensitive emulsions, improved fineness of grain and improved utilization of silver are obtained in the preparation of photographic emulsions by a procedure which comprises adding a silver nitrate solution to a relatively small portion of a halogen salt solution, to which gelatin has been added and which has been heated to an appropriate tempera- ;ture for the desired emulsion sensitivity, that is moved or agitated at a relatively high speed with respect to the rest of the halogen salt solution while the solution is in motion.

PATENTEDBEB21 1971 I 3,628,959

sum 1 OF 4 FIG.

PATENIEUnzazmn 3,628,959

SHEET 4 BF 4 FIG. 5

PROCESS FOR THE PREPARATION OF PHOTOGRAPHIC EMULSION This invention relates to a process for the preparation of photographic emulsions and apparatus for carrying out such processes.

Photographic emulsions are generally prepared by adding gelatin to halogen salts dissolved in water and heating the mixture, after swelling of the gelatin, to the temperature necessary to achieve the desired film sensitivity. An aqueous silver nitrate solution is then heated to a suitable temperature and used for the subsequent silver halide precipitation process.

The silver nitrate solution is permitted to flow into the halogen-salt solution, accompanied by continuous stirring to produce the light-sensitive silver bromide. The speed of rotation of the stirrer is restricted because above a certain speed, foaming occurs which is considered undesirable.

When using this known and relatively simple method, it is only in general possible to obtain good quality emulsions where large quantities e.g. at least 50 l. of emulsion are produced. Thus emulsions produced in commercial quantities for the preparation of marketable films are generally produced in quantities of 500 l. or more. It is believed that by operating in this way, the known method provides optimum results.

I have now found by studying the behavior of the ingredients of the emulsions that in the emulsions prepared according to conventional processes only about 50 percent of the silver salt used is utilized. Thus use is not made of about half of the expensive silver halide in the emulsion. l have further found that the proportion of the unused silver component is greater the higher the sensitivity of the emulsion. This is attributed to the following reasons 1. If the emulsion is prepared as described above by the known process, the silver halide forms granular agglomerates which reduce the density and greatly increase the granularity.

2. The gelatin, which serves as the protective colloid during the precipitation, decomposes in the undissolved state to a greater extent the higher the temperature and the longer complete precipitation takes. This results in an inferior light quantum utilization.

If one were able completely to utilize the silver nitrate used in the preparation of the emulsion, 40 to 70 percent of the silver, depending on the sensitivity of the film, could be saved. Due to the high cost of silver, this would represent a decisive advance in photographic chemistry.

The photographic industry for many decades has attempted to produce sensitizers having maximum efficiency. Sensitizers are generally complicated organic compounds which require specialists for their production and the more efficient compounds are frequently more expensive. The efficiency of the sensitizers is reduced to a greater or lesser extent by impurities, in particular by traces of metals. Optimum adsorption by the agglomerates is not however achieved due to the reduced surface area.

The object of the present invention is therefore not only to improve the above-described known process so as to prepare more sensitive emulsions, but also to improve the fineness of grain and the resolution in the films and to ensure improved utilization of the silver supplied to the gelatinsalt solution. The amount of silver which must be used may thus be reduced and an adequate density obtained, resulting in a lowering of manufacturing costs, quite apart from the increase in the efficiency of the sensitizing dyes. Furthermore, the washing procedure may be shortened with a saving of water.

According to the present invention therefore there is provided a process for the preparation of aqueous gelatino silver halide photographic emulsions in which an aqueous halogen salt solution is heated to the temperature required to produce a desired emulsion sensitivity, dry gelatin or water swollen gelatin at ambient temperature is added to the heated solution and a solution of silver nitrate is then added to the halogen-salt solution whilst the said halogen-salt solution is in motion whereby finely divided silver halide is precipitated to form the emulsion, the halogen salt in the solution from which the silver halide precipitates being at least in part potassium bromide.

According to a further feature of the invention there is provided apparatus adapted for use in processes according to the invention which apparatus comprises at least one container for halogen-salt solution, means associated with the said container for moving a portion of halogen-salt solution therein at relatively high speed either whilst remaining within the said container or whilst travelling along a passage connected thereto, and inlet means'for adding silver nitrate solution to the said portion of halogen-salt solution whilst the said portion is moving at relatively high speed whereby finely divided silver halide is precipitated therefrom.

The use of this process in general results in a considerably greater utilization of the silver due to the precautions taken in the handling of the gelatin, at least up to the commencement of precipitation. The gelatin is thus maintained at ambient temperature until addition of the heated halogen-salt solution and little or no decomposition of the gelatin takes place.

Bacterial attack on the emulsions has previously been inhibited by the use of agents which at least retard the decomposition of the gelatin. These agents have, for example, been bactericides e.g. a phenol, which have been added to the emulsions.

According to a preferred feature of the process according to the present invention, a bactericide or other such agent is incorporated into the gelatin, swollen gelatin, halogen-salt solution and/or silver nitrate solution. With the use of a bactericide or other such agent in this way, a particularly mild treatment of the gelatin results, and accordingly the gelatin is especially effective as a protective colloid during the silver halide precipitation.

If the bactericide is added to the silver nitrate solution, a submicroscopic silver separation may take place depending on the degree of action. During the precipitation process this extremely finely dispersed silver then forms nuclei of crystallization, which favor individual grain formation and the trapped nuclei act as sensitivity centers. These agents should however, be limited to those which at least do not increase the tendency to fog formation.

The use of lactose as a fog rate reducing agent in conjunction with increased emulsion density is known I (DBP 1,153,990). It has not hitherto been known, however, that the reduction of the silver during the precipitation process is necessary in order to obtain fine-grain emulsions and for increasing the sensitivity.

It has further been found that to obtain optimum dispersion it is desirable not only to mix the individual constituents intensively but also to ensure that the silver nitrate solution meets the halogen-salt solution at maximum speed difi'erential. This is important because the greater the speed differential the better the dispersion. ln operating the process according to the invention, it is thus preferred that only a relatively small portion of the halogen-salt solution should be moved at relatively high speed and also that the silver nitrate solution should be fed into the said portion of the halogen-salt solution again at relatively high speed. Under these conditions, despite the increased stirring action, no serious foaming problems need arise and agglomerate formation can be avoided.

According to a still further feature of the present invention, a further improvement in the sensitivity of the emulsion can be achieved without increasing the grain size when using potassium iodide in addition to potassium bromide as halide salt if the potassium iodide is not dissolved along with the potassium bromide in the halogen-salt solution. in this case, advantageously a concentrated potassium iodide solution is prepared and mixed with the solution of silver nitrate and then with the relatively small portion of the remaining salt solution which is moving at a relatively high speed. Thus silver iodide is initially fonned in high concentration. Alternatively, the concentrated potassium iodide solution may also be mixed with a concentrated potassium bromide solution and this concentrated solution introduced into the relatively small portion of the halogen salt solution which is moving at a relatively high speed.

The apparatus according to the invention is adapted for car-- rying out the process according to the invention. The apparatus may for example be a modification of conventional apparatus and consist of a tank and a rotator such as a stirrer, propeller, turbine or the like which may be inserted into the tank to agitate the solution in said tank. This known apparatus is modified according to the present invention to provide a means arranged in the working area of the rotator to increase the speed of movement of the solution which is moved by the rotator. This is achieved by the relationship between the position of the mouth of the inlet feeding in the silver nitrate solution and the means for increasing the speed of movement. It is recommended that a nozzle be used or a constriction of the flow of liquid be caused by the rotator. This nozzle may, for example be in the shape of a Venturi nozzle or may consist of a plate, body or the like covering the rotator on at least one side and having an orifice therein. it is also possible for exampie to provide a plurality of means for increasing the speed of movement of the liquid moved by the rotators.

The apparatus may also be constructed in some other way, provided that the part of the liquid to be moved is moved at a considerably higher speed than the rest of the liquid. The apparatus may therefore consist of a plurality of containers for the salt solutions. These containers may be pressurized and connected by a least one pipe and furthermore there should be provided a container for the silver nitrate solution, the con tainer being connected via in each case at least one pipe with the other pipes.

The greater the speed differential between the halogen-salt solution and the silver nitrate solution flowing into this pipe, the greater the possible reduction in gelatin content. The viscosity of the solution is therefore decreased making higher speeds possible. A lower gelatin content also results in a greater sensitivity in the: precipitated silver bromide. Fine-- grained emulsions, having almost exclusively individual grain formation and ofhigh sensitivity, can be obtained by the use of bactericidal solutions. Thus, for example DIN 27 films produced by the conventional process have almost exclusive agglomerated grains.

An emulsion prepared according to the process of the invention and having the same sensitivity as a conventionally prepared emulsion has a grain size about 75 percent of normal. This process makes it possible to produce highly sensitive films of previously unobtainable fine-grain form.

If a concentrated potassium iodide solution or a concentrated solution of potassium iodide and potassium bromide is mixed with the: relatively small portion of the salt solution which is moving at a relatively high speed at the same time as the silver nitrate solution then only one pipe is required for the supply of this concentrated solution.

ln order to bring about the intensive movement of a rela tively small initial volume of a halogen-salt solution, the containers, tanks, etc. have previously been provided with dished bases, the deepest point of which was on the axis of the container or the like. A stirrer introduced in the center of the solution has the disadvantage that for maximum immersion the total volume of liquid is given a rotary movement. This rotation is to be avoided, for example, by laterally displacing the stirrer. This necessitates a reduction in the depth of immersion of the stirrer in the solution and necessarily a reduc tion of the speed of rotation of the stirrer to avoid foaming.

Stainless steel vessels, e.g. chromium nickel steel vessels, have been used in conventional processes and they have the advantage of good thermal conductivity but suffer from the disadvantage of being more or less strongly corroded during the emulsion process. This leads to a contamination of the emulsion by traces of metals. These contaminants reduce the stability, the sensitivity and the spectral response. It is therefore proposed that the inner surface of each tank, container or the like be constructed of a corrosion-resistant material such as enamel, gold, wood, plastic, platinum, porcelain or the like to overcome this disadvantage. The necessity for polishing the chemically corroded tank is then eliminated and the emulsion can no longer acquire metal contaminants in this way. Obviously it is advisable to make the surfaces of the other parts of the tank which come in contact with the solutions such as rotators, nozzles, plates, pipes or the: like associated with the tanks, containers or the like of corrosion-resistant materials. Care must be taken to ensure that these corrosion-resistant materials do not absorb substances from the emulsion or liberate contaminants into the emulsion.

Emulsions prepared under these conditions, which are free from impurities, require considerably shorter washing times and therefore save water. The: intensive movement of a relatively small initial volume of the halogen-salt solution, effected as described above, requires the containers, tanks, or the like to have a dished base, the deepest point being on the axis of the container or the like. A stirrer introduced into the center of the solution to provide maximum penetration into the liquid suffers from the disadvantage of making the total volume of liquid move in a pronounced rotary fashion, result ing in a uniform circulation. Lateral displacement and sloping of the stirrer provides a better stirring action. The danger of foaming is increased, however, because of the necessarily reduced depth of immersion of the stirrer. This also applies to containers or the like having a flat base. If as is further proposed according to the invention it is ensured that the deepest point of the base is arranged eccentrically with respect to the feed opening, considerably better conditions are obtained and working may commence using smaller starting volumes.

An indication of the preferred quantities of the components used and how they are brought into effective contact with one another will be given in the following examples before discussing the apparatus which may be used to carry out the process according to the invention.

EXAMPLE 1.

For an emulsion with increased density (factor 2) Component A 25 ml. of H,0 5 ml. ol'a l0% bactericide solution e.g phenol or the like 30 g. of gelatin swelling for 30 minutes at 20 C. Component 8 1,500 ml. of H,0 400 g. of KBr 10 g. of Kl heating to 50' C. Component C.

5.000 ml. of H,0 500 g. of AgNO, heating to 45 C. Component D 500 ml. ofdry gelatin Component A is added to suitably agitated component B. Component C is then immediately permitted to run into the mixture, uniformly and over 4 minutes. Component D is then stirred in.

EXAMPLE 2 For an emulsion with increased density (factor 1) Component A 50 g. of dry gelatin L500 ml. of H,0 400 g. of KE-r IS g. of Kl heating to 50 C. Component C.

5,000 ml. olH,0 500 g. of AgNO, hcating to 43 C Component C.

500 g. of dry gelatin Component A is added to suitably agitated component B. Component C is then immediately permitted to run into the mixture over minutes. Component D is then stirred in.

The emulsions of examples 1 and 2 save 50 percent of the silver of normal emulsions. The densities of these emulsions are the same as those of emulsions prepared according to con ventional processes. This is due merely to the careful treatment of the gelatin.

EXAMPLE 3.'

For an emulsion with increased sensitivity and finer grain to be mixed as explained in example 1.

EXAMPLE 4.

For an emulsion with increased sensitivity and liner grain Component A 25 ml. of l-l,0 3 ml. ofa bactericide solution, e.g. phenol or the like 30 g. of gelatin. Component B [,500 ml. oft-[,0 400 g. of AgNO 17 g. of Kl heating to 55 C. Component C.

4,900 ml. ot'l-l,0 500 g. of AgNO, heating to 48 C. Component D 50 ml. ola 5% bactericide solution e.g. phenol Component E 500 g. of dry gelatin.

When the desired temperatures are reached, component B is suitably agitated and component A added. Simultaneously, component D is added to component C which is then permitted to flow uniformly into component B over 3 minutes. These emulsions contain separate grains only. The grain is finer and the sensitivity is considerably greater than that of equally sensitive commercial films. Their spectral response is improved and their stability increased.

EXAMPLE 5.

For a highly sensitive (emulsion) with finer grain Component A as in example I. Component B 1,400 ml. of H,o

400 g. of KBr l7 g. of Kl 50 ml. of a l0% hactericide solution e.g. sodium pentachlorophenoxide or p-chlorornetacresol. heated to 65 C. Component C 4,000 ml. of H,0 500 g. of AgNO, to be heated to 60 C. Component D as in example I.

Components B and C are heated to the desired temperatures, component B is suitably agitated, component A added and part component C is then immediately permitted to flow into the mixture over 6 minutes. Component D is then stirred The highly sensitive emulsion prepared according to example 5 has the same advantage as the emulsions prepared according to examples 3 and 4. The density of the emulsions of commercial, high-sensitivity films falls with increase in sensitivity. In order to compensate for the fall in density, the silver content, which is already fairly high, has previously been increased further without providing the most advantageous density. The density of the emulsion prepared according to example 5 is considerably better than that of the above-mentioned commercial films. Due to the finer grain and the improved density, it is possible to provide films with an even higher sensitivity than before.

EXAMPLE 6 For an emulsion with an even further increased sensitivity without increased grain Component A 25 ml. of H,0 5 ml. of a l0% bactericide solution 30 g. otgelatin to swell for 30mins. at 20 C. Component 8 1.400 ml. of H,0 50 ml. of a 10% bactericide solution,

400 g. of KBr temperature 65 C. Component B,

17 g. of Kl, 50 ml. of H 0, temperature 65 C. Component C 4,000 ml. of H 0, 500 g. of AgNO, temperature 60 C. Component D 500 g. of dry gelatin After components B, B, and C have reached the abovementioned temperatures, component B is suitably agitated in the vicinity of the rotator, component A is added to component B and immediately thereafter component B, and component C are allowed to flow into the mixture i.e. component B over 60 secs. and component C over 6 mins. Component D is then stirred in.

it is also advantageous to introduce concentrated potassium bromide solution at the same time and in addition to the concentrated potassium iodide solution.

EXAMPLE 7.

For an emulsion with increased sensitivity without increased grain Component A 25 ml. of H,0 5 ml. ofa IOl: bactericide solution. e.g. phenol or the like 30 g. of gelatin to swell for 30 mins. at 20 C. Component B nun-e "Ann L400 ml. of H4) 50 ml. ofa I: baictericide solution 330 g. of KBI'.

temperature 65' C. Component 8,

l7 g. of Kl 70 g. of KBr I60 ml. of H,0

temperature 65' C. Component C 4,000 ml. of H,0

500 g. of AgNO,

temperature 60 C. Component D 500 g. of dry gelatin After components B, B, and C have reached the desired temperatures, component B is suitably agitated in the vicinity of the rotator, component A added to component B and immediately following components B, and C are allowed to flow into the mixture i.e. component 8, over 60 secs. and component C over 6 mins.

The subsequent steps following this emulsion preparation such as washing, secondary maturing and sensitization may be effected according to conventional process steps which need not be changed.

The apparatus which may be used for carrying out the process according to the invention will now be explained in more detail with reference to the accompanying drawings wherein:

FIG. 1 is a vertical section through an open boiler having an eccentrically offset base with an outlet therein; a rotator with an associated additional member to provide a considerably increased localized agitation and a feed pipe for the silver nitrate solution, which terminates in the center of the additional member,

FIG. 2 is a vertical section through a. boiler with an eccentrically arranged, cylindrical base and a multistage rotator, together with a feed pipe for the silver nitrate solution,

FIG. 3 is a vertical section through a venturi tube with two feed pipes connected for delivery of the silver nitrate solution,

FIG. 4 is a vertical section through a feed device comprising a rotator and a nozzle;

FIG. 5 is a vertical section through two containers which can be pressurized alternately and which are connected by a pipe which is connected to a third container by a further pipe; the container may be filled with a silver nitrate solution, and

FIG. 6 is a vertical section of a further embodiment of apparatus according to the invention with an annular pipe.

The boiler I which is open at the top and is shown in FIG. 1 has a base asymmetrically the lowest portion 3 of which is ar ranged asymmetrically with respect to the filling inlet 8. The inner surface 1' consists of a corrosion-resistant material, such as enamel, glass, wood, a synthetic resin, porcelain or the like. Above the lowest portion .3 of the boiler, rotator 5 is provided constructed as a stirrer, and driven by a shaft 6. .A plate 8 hav ing an orifice 7 is nonrotatably mounted and supported, for example in relation to the inner wall of the boiler as indicated at 8. In the region of the lowest portion 3, a discharge valve 9 is provided, which however need not be used since the boiler can also be emptied by tilting. A feed pipe 11 from a container terminates in the area of the orifice 7 of the plate 8. The required halogen salt solution is prepared in the boiler I or the latter filled therewith, the liquid level is shown at 12. The rota tor 5 is rotated, the locking valve 13 opened and the silver nitrate solution stored in the container 10 is fed to the area in the boiler i at which a relatively small portion of halogen salt solution is moving at a relatively high speed. Intensive mixing of the halogen salt solution with the silver nitrate solution to precipitate the silver, takes place. The liquid level 14 of the silver nitrate solution indicates that the quantities of the two solutions are not equal as with ID percent silver nitrate solution, but for example l:5. The silver nitrate solution is con tinuously fed into the liquid in boiler 1, an extremely intensive mixing taking place, until the whole of the silver nitrate solution is in boiler 1.

The boiler 15 in FIG. 2 is likewise so constructed that its lowest portion 3' is arranged asymmetrically with respect to the filling aperture. The boiler has a well 16 in which are situated two coaxial and separated rotators 17 and 18 constructed as turbines, each of which is provided with av plate 8 having an orifice 7. In the embodiment shown, these plates are interconnected by a hollow cylinder 19, and are nonrotatably secured as is the plate 8 in FIG. 1. The feed pipe 11 terminates in the vicinity of the opening 7 of the lower plate 8 and leads from a container HI (not shown). The working procedure corresponds to that explained for FIG. I and an even higher mixing speed can be obtained which can be further increased by providing further rotators and plates. A venturi nozzle 21 (FIG. 3) may be used in place of the plate 8 provided with the orifice. Feed pipes 11 and Ill terminate in the vicinity of the narrowest cross section of this venturi nozzle or after it.

Another possibility of supplying the silver nitrate solution is shown in FIG. 4. A turbine 25 in a recess 24 in a nozzle body 22 is provided with a bottleneck 23; both feed pipes 11 and 11' for the silver nitrate solution terminate in the vicinity of the bottleneck 23, although they may terminate at other positions.

The arrangement shown diagrammatically in FIG. 5 consists of two boilers 26 and 27 which are interconnected by a pipe 28. The feed pipe 11 which is connected to a container 10' which can be indated by a valve 13 terminates at .29. The closed containers 26 and 27 are then connected by alternately operable valves 30 and 31 to a compressed air source 32. The initial quantity of a prepared halogen salt solution is used to fill the boiler 26, for example through a funnel 33, behind which a locking valve 34 is arranged, A corresponding device 34' is associated with the boiler 27, It is recommended that at least one of the two boilers 26 and 27 be provided with an outlet 35 which may be closed by the valve 36. Valves 37 and 38 may also be provided.

If compressed air is introduced into the boiler 26 with valve 30 and valve 34 closed, the halogen-salt solution in the boiler 26 flows into the boiler 27, when the valves 37 and 38 are open, silver nitrate being added to this halogen-salt solution when the valve 13 is open. The valve 34 must be open to enable the satisfactory transfer of the halogen-salt solution into the boiler 27. It must be ensured that silver nitrate solution is added when transferring the solution from boiler 26 into boiler 27 or from boiler 27 into boiler 26. Valves 30 and 34 are closed and valves 31 and 34 are opened. The halogen-salt solution mixed with some silver nitrate solution and present in the boiler 27 then is forced through the pipe 28 into the boiler 26, more silver nitrate being added. This procedure is repeated a number of times until all the silver nitrate solution in container III! is added to the halogen-salt solution. The two boilers 26 and 27 must be of such a volume that they can each contain the combined halogen-salt and silver nitrate solutions. The emulsion can be discharged for further processing by opening the valve 36. If desired, the valves used with this embodiment of the invention may for example be automatically controlled and be arranged to operate in the sequence required to operate the process according to the invention.

FIG. 6 shows a boiler 39 which is partially filled with a halogensalt solution. The boiler containing the silver nitrate solution is shown at 40. A pump 41 is connected to the boiler 39 by a suction pipe 42 and a pressure pipe 43. A pipe 44 serves to supply the silver nitrate solution and it is connected by a pipe 45 to the suction pipe 42. It is further connected by a pipe 46 to the suction side of the pump 41 and by a pipe 47 to the pressure pipe 43. In each case, connection is such as to permit the silver nitrate solution to flow into the boiler. Due to the high speed of the liquid in the pipes 42 and 43, a satisfactory mixing of the halogen-salt and silver nitrate solutions is effected. The mixing speed may be increased by providing one or more constrictions.

Iclaim:

I. A process for the preparation of an aqueous gelatino silver halide photographic emulsion which comprises heating an aqueous halogen-salt solution to the temperature required to produce a desired emulsion sensitivity, adding dry gelatin or water-swollen gelatin at ambient temperature to the heated solution, moving a relatively small portion of the total halogen-salt solution at a relatively high speed, and adding a solution of silver nitrate to said relatively small portion of the halogen-salt solution while the halogen-salt solution is in motion, whereby finely divided silver halide is precipitated to form the emulsion, the halogen salt in the solution from which the silver halide precipitates being at least in part potassium bromide.

2. A process as claimed in claim 1 in which at least one bactericide which inhibits degradation of gelatin is combined with the halogen-salt solution, the dry gelatin, the water-swollen gelatin or the silver nitrate solution.

3. A process as claimed in claim 1 in which a concentrated solution of potassium iodide is added to the said portion of the halogen-salt solution simultaneously with the addition of the silver nitrate solution.

4. A process as claimed in claim 1 in which a maximum difparts by weight of gelatin, 400 parts by weight of potassium bromide, l0 to l7 parts by weight of potassium iodide and 500 parts by weight of silver nitrate.

8. A process as claimed in claim 1 in which phenol is added to the gelatin solution as a bactericide.

9. A process as claimed in claim 1 in which phenol is added to the silver nitrate solution and to the gelatin solution as a bactericide.

* i ll t 

2. A process as claimed in claim 1 in which at least one bactericide which inhibits degradation of gelatin is combined with the halogen-salt solution, the dry gelatin, the water-swollen gelatin or the silver nitrate solution.
 3. A process as claimed in claim 1 in which a concentrated solution of potassium iodide is added to the said portion of the halogen-salt solution simultaneously with the addition of the silver nitrate solution.
 4. A process as claimed in claim 1 in which a maximum differential of speed is maintained between the motion of said halogen-salt solution and the silver nitrate solution added thereto.
 5. A process as claimed in claim 1 in which potassium iodide is also added to said portion of halogen-salt solution that is moved at a relatively high speed.
 6. A process as claimed in claim 1 in which said aqueous halogen-salt solution is heated to a temperature of from 40* to 65* C.
 7. A process as claimed in claim 1 in which the total amount of components added to the emulsion comprises 515 to 550 parts by weight of gelatin, 400 parts by weight of potassium bromide, 10 to 17 parts by weight of potassium iodide and 500 parts by weight of silver nitrate.
 8. A process as claimed in claim 1 in which phenol is added to the gelatin solution as a bactericide.
 9. A process as claimed in claim 1 in which phenol is added to the silver nitrate solution and to the gelatin solution as a bactericide. 